The systems and methods described here relate to systems and methods for producing crystals.
Materials grown in single crystal form in an as-grown state are typically referred to as boules, while materials in multicrystalline form are referred to as multicrystalline ingots. At times, boules and ingots are collectively referred to as crystals. In this document, the term “crystals” is intended to include at least boules and ingots collectively, and, in some instances, boules and ingots may be referred to separately to demonstrate distinctions between single crystal and multicrystalline materials.
For crystal growth in some systems it can be desirable to grow crystals, such as sapphire or silicon, from the bottom to the top of a crucible that holds a molten material. The bottom of the furnace should therefore be cooler than the top, preferably with a stabilizing temperature gradient that minimizes convection and avoids constitutional supercooling. The material in the crucible can solidify from the bottom to the top due to the vertical gradient. This process helps to achieve directional solidification and thereby rejection of impurities to the melt for impurities having a segregation coefficient of less than 1 (very rarely do impurities have a segregation coefficient greater than 1. The process thereby produces a purer solid. In case of silicon, the segregation coefficient of Fe is 10−6 and for refractory metals it is even less than 10−9; consequently, directional solidification can be an effective purification process. For reactions with the melt resulting in volatile product, the gases can be rejected upwardly through the melt so it can escape.